Multi-stage astable multivibrator

ABSTRACT

A multi-stage astable multivibrator wherein a plurality of astable multivibrators each consisting of two transistors are connected in parallel so that a high output may be derived and a completely rectangular output waveform may be obtained. Diodes are connected to the emitters of the transistors so that the breakdown of the transistors may be prevented.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-stage astable multivibrator.

In the conventional astable multivibrators, the feedback circuits of the two transistors are capacitively coupled so that the transistors may be alternately conducted and cut off. However, the reverse-breakdown voltage between the base and emitter of the transistor is generally so low that there is a fear that the transistor is degraded or broken down when the high reverse voltage is applied thereto. Furthermore, the output is low, and the output waveform has the rounded rising side so that the completely rectangular output waveform cannot be obtained.

SUMMARY OF THE INVENTION

In view of the above, one of the objects of the present invention is to provide a multi-stage astable multivibrator of the type comprising a plurality of or at least two astable multivibrators each consisting of two transistors, whereby the high output may be derived.

Another object of the present invention is to provide a multi-stage astable multivibrator which may produce the substantially completely rectangular output waveform by the sequential conduction of the transistors.

A further object of the present invention is to provide a multi-stage astable multivibrator in which diodes are connected to the emitters of the transistors so that the breakdown of the latter may be prevented.

Briefly stated, according to the present invention, a first and a second transistors have their collectors connected to the bases of the second and first transistors, respectively, through a parallel circuit consisting of a resistor and a speed-up capacitor, the bases of the first and second transistors being directly connected to a power supply; and the emitters of the first and second transistors are connected directly or through a diode to the collectors of a third and a fourth transistors, respectively, the bases of the third and fourth transistors being connected through a capacitor to the collectors of the fourth and third transistors, respectively, the bases of the third and fourth transistors being connected through a resistor to the power supply.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit diagram of a conventional astable multivibrator;

FIG. 2 shows the waveforms used for the explanation thereof; and

FIGS. 3 and 4 are circuit diagrams of a first and a second embodiments of the present invention, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT Prior Art, FIGS. 1 and 2

Prior to the description of the preferred embodiment of the present invention, the prior art astable multivibrator will be briefly described with reference to FIGS. 1 and 2. The astable multivibrator comprises two transistors Q₁ and Q₂, collector resistors R_(C1) and R_(C2), resistors R₁ and R₂ and capacitors C₁ and C₂ which determine the time constants. FIGS. 2A and 2B show the voltage waveforms at the base and collector, respectively, of the transistor Q₁ while FIGS. 2C and 2D, the voltage waveforms at the base and collector, respectively, of the transistor Q₂. Assume that the transistor Q₁ conducts. Then, the base voltage A and the collector voltage B of the transistor Q₁ are V_(BES1) and B_(CES1), respectively, of FIGS. 2A and 2B, and the collector voltage D of the transistor Q₂ is Ec while the base voltage C, a certain negative value at which the transistor Q₂ is cut off. The capacitor C₁ is charged through the resistor R₁ with the supply voltage Ec, the base potential C of the transistor Q₂ rises exponentially and reaches V_(BES2) shown in FIG. 2C, at which the transistor Q₂ conducts. Immediately before the transistor Q₂ conducts, the base potential A of the transistor Q₁ is V_(BES1) while the collector potential D of the transistor Q₂, at Ec. Therefore, when the transistor Q₂ conducts so that the collector potential D suddenly drops to V_(CES2), the base potential of the transistor Q₁ drops from V_(BES1) to -Ec + V_(BES1) + V_(CES2) so that the transistor Q₁ is cut off. The collector potential B of the transistor Q₁ rises to Ec while the base potential rises from -Ec + V_(BES1) + V_(CES1) toward Ec exponentially with the time constant dependent upon the values of the capacitor C₂ and the resistor R₂. When the base potential reaches V_(BES1), the transistor Q₁ conducts while the transistor Q₂ is cut off. Thus the transistors Q₁ and Q₂ are alternately conducted and cut off; that is, the astable multivibrator oscillates. However, in general, the reverse-breakdown voltage between the base and emitter of the transistor is considerably low so that the high reverse-breakdown voltage of -Ec + V_(BES1) + V_(CES2) causes the degradation and breakdown of the transistor. Furthermore, the output is low, and the output waveform has the rounded rising side so that the complete rectangular waveform cannot be obtained.

The Invention

First Embodiment, FIG. 3

Next referring to FIG. 3, the first embodiment of the multi-stage astable multivibrator in accordance with the present invention will be described. The collectors of transistors Q₃ and Q₄ are connected through loads Z₁ and Z₂ to the power supply +Vc, and the collector of the transistor Q₃ is connected to the base of the transistor Q₄ through a parallel circuit consisting of a capacitor C₃ and a resistor R₃ while the collector of the transistor Q₄ is connected to the base of the transistor Q₃ through a parallel circuit consisting of a capacitor C₄ and a resistor R₄. The emitters of the transistors Q₃ and Q₄ are connected to the collectors, respectively, of transistors Q₅ and Q₆. The collector of the transistor Q₅ is connected to the base of the transistor Q₆ through a capacitor C₅ while the collector of the transistor Q₆ is connected to the base of the transistor Q₅ through a capacitor C₆. The bases of the transistors Q₅ and Q₆ are also connected through resistors R₆ and R₅, respectively, to the power supply +Vc. The emitters of the transistors Q₅ and Q₆ are grounded.

Next the mode of operation of the astable multivibrator with the above configuration will be described. Assume that the transistors Q₃ and Q₅ are conducted. Then the potential at the point A or the collector voltage of the transistor Q₃ is at V_(CES3) + V_(CES5) while the potential at the point B or the collector voltage of the transistor Q₅ is at V_(CES5). The base potential of the transistor Q₅ is at V_(BES5) while the base potential of the transistor Q₆, at a certain negative potential. The potential at the point E or the collector potential of the transistor Q₄ is at +Vc. The negative potential at the point D cuts off both the transistors Q₄ and Q₆. When the voltage across the capacitor C₅ rises so that the potential at the point D also rises to V_(BES6), the transistor Q₆ conducts while the transistor Q₅ is cut off so that the emitter potential of the transistor Q₃ rises to +Vc. As a result the transistor Q₃ is also cut off while the transistor Q₄ conducts as the emitter potential of the transistor Q₄ drops to V_(CES6) as the transistor Q₆ conducts. The potential at the point E is V_(CES6) + V_(CES4) while the potential at the point C is -Vc + V_(BES5) + V_(CES6). Therefore both the transistors Q₃ and Q₅ are cut off. The base current flows through the load resistor Z₁ and the resistor R₃ into the transistor Q₄ so that the latter is conducted. The conduction of the transistor Q₄ occurs very rapidly because the speed-up capacitor C₃ is connected in parallel with the resistor R₃. As the transistor Q₆ conducts, the potential at the point C rises expotentially and reaches V_(BES6), at which the transistor Q₅ conducts while the transistors Q₄ and Q₆ are cut off. As described above, the transistors Q₄ and Q₆ and the transistors Q₃ and Q₅ are alternately conducted and cut off.

V_(CES3), V_(CES4), V_(CES5) and V_(CES6) are the saturation voltages between the collector and emitter of the transistors Q₃ to Q₆, respectively; V_(BES3), V_(BES4), V_(BES5) and V_(BES6), the forward voltages from the base to emitter of the transistors Q₃ -Q₆, respectively.

The multi-stable astable multivibrator of the type described has a high output and a very short rise and fall time because the speed-up capacitors C₃ and C₄ are inserted. Unlike the prior art astable multivibrator, the output terminals E and A are not connected through capacitors to the base of the transistors and are not connected to the integrators which in turn are connected to the power supply, the rise and fall time are very short so that the complete rectangular output waveform may be derived.

Second Embodiment, FIG. 4

The second embodiment shown in FIG. 4 is substantially similar in construction to the first embodiment described so far with reference to FIG. 3 except that diodes D₁ -D₄ are connected to the emitters of the transistors Q₃ -Q₆, respectively; resistors R₇ and R₈ are inserted between the base and emitter of the transistors Q₅ and Q₆, respectively; and resistors R₉ -R₁₂ are interconnected between the collectors of the transistors Q₃ -Q₆ and the cathodes of the diodes D₁ -D₄.

Next the mode of operation of the second embodiment with the above configuration will be described. Assume that the transistors Q₃ and Q₅ conduct. Then, the potential at the point A is V_(D3) + V_(CES5) + V_(D1) + V_(CES3) ; the potential at the point B, V_(D3) + V_(CES5) ; the potential at the point E, +Vc; the potential at the point F, 1/2Vc; and the potential at the point D, at a certain negative value, at which the transistors Q₄ and Q₆ are cut off. When the capacitor C₅ is charged so that the potential at the point D rises to V_(D4) + V_(BES6), the transistor Q₆ conducts so that the potential at the point F drops from 1/2Vc to V_(CES6) while the potential at the point C changes from V_(D3) + V_(BES6) to -1/2Vc + V_(BES6) + V_(D4). As a result, both the transistors Q₃ and Q₅ are cut off so that the base current flows through the load Z₁ and the resistor R₃ into the transistor Q₄. The voltage across the capacitor C₆ rises exponentially toward Vc and when the potential at the point C rises to V_(BES5) + V_(D3), the transistor Q₅ conducts while the transistors Q₄ and Q₆ are cut off and the transistor Q.sub. 3 conducts. The V_(BES3) -V_(BES6) and V_(CES3) - V_(CES6) have been already explained above, and V_(D1) -V_(D4) are the forward voltages of the diodes D₁ -D₄, respectively.

The insertion of the diodes D₁ -D₄ and the resistors R₉ -R₁₂ may attain the following features. When the transistors Q₃ and Q₅ are cut off, the voltage from the collector to emitter of the transistor Q₃ is R₉ Vc/(R.sub. 9 + R₁₁) while the voltage from the collector to emitter of the transistor Q₅ is R₁₁ Vc/(R₉ + R₁₁). When the transistors Q₄ and Q₆ are cut off, the voltage from the collector to emitter of the transistor Q₄ is R₁₀ Vc/(R.sub. 10 + R₁₂) and the voltage from the emitter to collector of the transistor Q₆, R₁₂ Vc/(R₁₀ + R₁₂). Therefore, the astable multivibrator may oscillate even when the supply voltage +Vc is high, but the following conditions must be satisfied:

    Z.sub.1 << R.sub.9

    z.sub.1 << r.sub.11

    z.sub.2 << r.sub.10,

and

    Z.sub.2 << R.sub.12.

even when the potential at the point B is 1/2Vc which is higher than the potential at the point E of V_(CES4) + V_(D2) + V_(CES6) + V_(D4), the transistor Q₃ is protected by the diode D₁. In like manner, even when the potential at the point F becomes +1/2Vc which is higher than the potential V_(CES2) +V_(D1) +V_(CES5) + V_(D3) at the point A, the transistor Q₄ is protected by the diode D₂. Furthermore, even when the reverse bias voltage of -1/2Vc + V_(BES5) + V_(D3) -1/2Vc + V_(BES6) + V_(D4) is applied to either the point C or D, the transistors, Q₅ and Q₆ are protected by the diodes D₃ and D₄. The resistors R₇ and R₈ have a double function of supplying the emitter-base current I_(EBO) to the transistors Q₅ and Q₆ and of stabilizing the oscillation of the astable multivibrator. Furthermore, the resistors R₇ and R₈ have a function of discharging the charge stored in these transistors. The resistors R₉ and R₁₁ are inserted to maintain the appropriate voltages between the collector and emitter of the transistors Q₃ and Q₅ while the resistors R₁₀ and R₁₂ maintain the suitable voltage between the collector and emitter of the transistors Q₄ and Q₆, whereby the oscillation may be stabilized. 

What is claimed is:
 1. A multi-stage astable multivibrator comprising(a) first and second NPN transistors with the collectors of said first and second NPN transistors connected to the bases of the second and first NPN transistors, respectively, through a parallel circuit consisting of a resistor and a capacitor, and (b) third and fourth NPN transistors with their collectors D.C. coupled to the emitters of said first and second NPN transistors, respectively, and with the collectors of said third and fourth NPN transistors connected to the bases of said fourth and third NPN transistors, respectively, through a capacitor and the bases of said third and fourth NPN transistors connected through resistors to a power supply.
 2. A multi-stage astable multivibrator comprising(a) first and second transistors with the collectors of said first and second transistors connected to the bases of the second and first transistors, respectively, through a parallel circuit consisting of a resistor and a capacitor; and (b) third and fourth transistors with their collectors connected to the emitters of said first and second transistors, respectively, through diode means, and with the collectors of said third and fourth transistors connected to the bases of said fourth and third transistors, respectively, through a capacitor, the bases of said third and fourth transistors connected through resistors to one terminal of a power supply, and the emitters of said third and fourth transistors, each being connected through diode means to another terminal of said power supply.
 3. A multi-stage astable multivibrator as set forth in claim 2 whereina resistor is inserted between the base and emitter of each of said third and fourth transistors.
 4. A multi-stage astable multivibrator as set forth in claim 2 whereinthe collectors of said first and second transistors are connected through resistor means to the collectors of said third and fourth transistors, respectively; and the collectors of said third and fourth transistors are grounded through resistor means. 